1. Field of the Invention
The present invention relates generally to controls for electric motors, and more specifically to a control system for controlling operation of a stepper motor.
2. Description of the Prior Art
The construction, control, and use of stepper motors is well known to those skilled in the art. Because of their design, stepper motors are able to accurately position the rotor at multiple known locations. Because of this capability, stepper motors have many uses. For example, they are used to position the platen and print head in electronic typewriters and printers.
Controllers for stepper motors are used to energize the field coil windings in selected sequences to change the position of the armature. Various switching sequences of the different field coil windings are used to give full step and half step positioning of the armature. Stepper motors can also hold the armature in a fixed position. This is typically referred to as detent mode.
Stepper motors typically have 2 independent field coils, but other numbers may be used. For example, it is known to construct stepper motors having 5 field coils. The number of field coils, and the number of armature pole pairs used, determine the angular separation between each full or half step position. For example, a motor having 50 armature pole pairs and 2 field windings may be positioned at 200 different settings in the full step mode. This corresponds to an angular separation of 1.8.degree.. Controlling this same motor in half step mode gives 400 positions, each 0.9.degree. apart.
A H-bridge is provided for each field coil. Switching of transistors in the bridge is used to cause current to flow in either direction through the coil as desired. The transistors can also be turned off so that no current flows from the power supply through the coil. Controlling the switching of the transistors for the two coils steps the motor in a forward or reverse direction as desired.
Each bridge has four power transistors, so that eight are required for a typical two coil motor controller. These power transistors may be bipolar junction transistors or field effect devices. Since power transistors must conduct large driving currents, on the order of one or two amps for a typical medium-sized stepper motor, they must be physically large. As known in the art, large devices result in a low level of integration, so that control functions must be designed as separate chips.
It is desirable to increase integration levels for integrated circuits generally. It would be desirable to increase integration levels for stepper motor control circuitry. It would be further desirable to provide a control system for a stepper motor which utilized a lesser number of power transistors, allowing increased levels of integration.